1. Field of the Invention
The present invention relates to fuel cells and, more specifically, to fuel cells employing ion exchange membranes.
2. Description of the Related Art
Among the various types of fuel cells, the proton-exchange membrane (PEM) fuel cell has several desirable features including well established membranes and cell designs. Although PEM fuel cells have been used in numerous applications, there are several obstacles that impede wide scale commercialization. These issues include the high cost of noble-metal catalysts and perfluorinated membranes, carbon monoxide poisoning, and limited lifetime due to membrane and electrode degradation.
Recently, the anion exchange membranes (AEM) have been employed to make anionic fuel cells. Although AEM technology is not yet as mature as PEM, AEM technology is promising because it could address several drawbacks with PEM fuel cell. The high pH environment in AEM fuel cell provides faster kinetics for both oxygen reduction and methanol oxidation, which allows non-Pt catalysts such as silver and nickel to be used. Also, methanol crossover is expected to be lower due to the opposite direction of electro-osmotic drag. The high pH environment also addresses many of the shortfalls experienced with PEM fuel cells. Alkaline cells can employ catalysts such as nickel and silver, rather than the considerably more expensive platinum used in most low pH fuel cells. Alkaline cells are also resistant to CO poisoning.
Although AEM fuel cells have several advantages compared to proton based fuel cells, the lower ionic conductivity of AEM's compared to commercially available PEM's (such as Nafion®) is a concern because it may lower the performance. Moreover, the strong dependence of the AEM conductivity on humidity and the need for water in the cathode reaction are significant challenges that limit the performance of current AEM fuel cells.
Another aspect of fuel cells is that they can have high energy density when liquid fuels are used. Direct methanol fuel cells (DMFCs) have several key advantages compared to other power sources. The high theoretical energy density of methanol (6100 Wh/kg at 25° C.) may lead to small volume, long-life sources. The passive DMFC system, operating at atmospheric pressure and ambient temperature (20° C. to 60° C.), has a simple design, high energy efficiency, and minimal balance of plant. In addition, the liquid fuel is easy to store and handle
In order to achieve higher voltage than values obtained from a single fuel cell, and high power-density, multiple fuel cells can be connected in series in a stack. Several different types of stack design for proton exchange membrane (PEM) fuel cell have been studied. The bipolar stack connects the anodes and cathodes in series through a metallic bipolar plate, which also serves as a fuel distribution channel. Another design is a monopolar stack where multiple anodes are serviced by the same fuel supply. The series connection is accomplished by electronically connected to the cathode of the next cell in a series configuration. Although it has attractive features, such as light weight and low cost, it was hard to achieve high power due to the high internal resistance. Moreover, in case of DMFC application, there is a concern about possible electrolysis of the water in the fuel, because more than 1.2V could be produced with several electrodes sharing the same fuel tank.
One system includes a bi-cell stack design (or pseudo bipolar), in which each unit consists of two PEM single cells. The two anodes (A1 and A2) operate with a common fuel source or channel, and the cathode (C2) faces the cathode (C3) in the next bi-cell unit. The anode (Ax) is electronically connected to the next cell's cathode (Cx+1) to form a series connection. It is easy to assemble the stack and the overall volume is smaller than the normal bi-polar stack due to the common fuel tank. Also, the bi-cell design reduces the need for expensive bipolar plates.
However, there is a potential difference between anode A1 and cathode C2. When these two electrodes are shorted together in the series configuration, the liquid methanol fuel provides an ionic path for anode A1 to act as the anode to cathode C2. Since A1 and C2 are electrically shorted, no electrical current flows in the external circuit as a result of this electrochemical reaction. Under acidic conditions, the standard potential for the two electrochemical reactions is given in Equations 1 and 2, respectively, and the overall reaction is given by Equation 3.Anode: CH3OH+H2O?6H++6e−+CO2 (E°a=−0.02 V vs. NHE at 25° C.)  (1)Cathode: 3/2O2+6e−+6H+?3H2O (E°c=1.23V vs. NHE at 25° C.)  (2)Overall: CH3OH+3/2O2?2H2O+CO2 (Ecell=1.21V vs. NHE at 25° C.)  (3)
Thus, the origin of this electrochemical short circuit between anode A1 and cathode C2 is field developed between the electrodes and ionic path through the liquid methanol. This results in a self-discharge mechanism and loss of fuel efficiency. This same short circuit can also occur in the monopolar stack, since the anode in one cell is shorted to the cathode in the next cell and the two are ionically connected through the common methanol fuel tank. The magnitude of the undesired proton transport through the fuel tank could be lessened by spacing the cells farther apart or forming an insulating barrier between adjacent cells; however, this would be at the expense of compact designs.
Therefore, there is a need for fuel cells that have high ionic conductivity that can also employ less expensive catalysts and resist CO poisoning.
There is also a need for fuel cells stacks that limit self-discharge.